The Arrive is a decent phone. I bought it the day it came out so I can comment on it pretty well. IMO the major problem with buying it today for the next two years is that it was already outdated a year ago-- three years from now it will be a real problem.
It's true that WP doesn't need the latest and greatest specs to run really well, but if superior specs are available on other carriers, why settle for less. It's not simply the processor speed-- it's the processor architechture, the graphics processor, the radio (4G/LTE), the screen quality, etc.
A major issue I've faced with the phone is it's just so damn bulky. Yes, I get an awesome qwerty keyboard but honestly I'd rather have a larger screen and make do with the virtual keyboard. Also I'm not a huge fan of the charging port being on the side of the device. On the plus side it's built like a tank, I've dropped it a few times (no case or screen protector) and it's no worse for wear.
If I were in the market for a phone today, the Arrive would be pretty low on my list. Not that it's a bad phone per se, but there are better ones out there. Just not at Sprint unfortunately.
-R
EDIT: Oh and it has that stupid, maddening 160 character limit for SMS. AFAIK it's the only windows phone that has that issue.